In an illumination optical system of an optical apparatus, such as a projection apparatus, an exposure apparatus, and a microscope, the ability to illuminate the entire illumination area uniformly and with sufficiently bright light is required.
Known illumination optical apparatuses for realizing such illumination include, for example, the illumination apparatus described in Japanese Unexamined Patent Application Publication No. 6-349710 (refer to the section from line 31 in column 10 to line 30 in column 11, as well as FIG. 7).
This illumination optical apparatus improves, for example, the power of illumination light and the illuminance uniformity in relation to beams emitted by a plurality of light sources by using a fly-eye lens. Furthermore, the illumination optical apparatus splits the beam from each light source with a beam-splitting optical system (half-prism) and makes the beams incident upon the fly-eye lens from different angles to prevent illuminance nonuniformity from occurring.
This illumination optical apparatus, however, is problematic in that, for example, it requires a beam splitting system, which causes the optical system to become complicated. The illumination optical apparatus has another disadvantage in that since high parts precision is required to achieve the desired performance, the apparatus becomes very costly. Furthermore, to efficiently receive at the fly-eye lens light emitted from the light sources while preventing, for example, vignetting, the shapes and the layout of the light sources and the shape of the fly-eye lens are subjected to more restrictions. In addition, a desirable structure is one in which the types, intensity, and the like of the light sources are selectable as required according to, for example, the properties of the specimen to be examined. However, such a structure cannot be employed in the above-described known illumination optical apparatus.